1. Field of the Invention
This invention relates generally to industrial trucks and more specifically to systems for controlling the operation of fluid actuated clutches in fluid power shifted transmissions employed in such trucks.
2. Description of the Prior Art
The operation of industrial lift trucks having fluid power shifted transmissions generally requires that provision be made for "inching" the vehicle. That is, the vehicle must be capable of moving very slowly while its engine operates at a high speed. This mode of operation is commonly effected in prior art systems by use of a pressure regulating valve that reduces the pressure of the fluid supplied the actuated clutch, causing it to slip, in response to increases in brake actuation force. See, e.g., U.S. Pat. No. 3,715,017.
The prior art systems, however, have exhibited certain disadvantages. One of these arises from the fact that the pressure regulating valve has been fluidly connected in series with clutches, causing the valve to operate on that portion of the outlet flow of the pump used for clutch actuation. This is a relatively low flow which varies with clutch leakage rate. Operating on this low, fluctuating flow makes control of clutch actuation pressure difficult, often causing "starving" of a clutch as slipping is effected and subsequent flow surges when the starved clutch is reactuated. These flow phenomena result in erratic operation of the vehicle in the inching mode.
Another related disadvantage is the number and complexity of components needed in prior art systems to control the rate of pressure buildup in the clutches during changes of mode of operation, i.e., from forward to reverse or from normal to inching operation. High rates of pressure change tend to cause the vehicle to lurch. Prior art systems, of which the previously cited U.S. Pat. No. 3,715,017 is exemplary, have solved this lurching problem by additional valve components coupled to the customarily used selector and pressure regulator valves of an inching control system.